It is known to produce single crystal metallic objects by casting. This is done in a mold which permits solidification to progress from one end of the mold toward the other end of the mold. At the part of the mold at which metal first solidifies there is a crystal selector structure which, by providing a tortuous path, causes metal which is freezing to become by competitive grain growth essentially single crystal metal by bending the freezing front around corners or around bends such as in a pigtail configuration. When the freezing front of metal enters the cavity proper of such a mold the metal is or should be freezing in the configuration of a single crystal. Thereafter, the freezing front is maintained in the principal cavity of the mold to produce single crystal objects such as turbine blades useful in the hot stages of gas turbine engines. U.S. Pat. No. 3,724,531 discloses such a method.
Such a method of producing single crystal casting is of no use when the alloy out of which the single crystal object is to be made is a metal which is hardened by a dispersion of a material which does not form a liquid phase in the molten alloy. To illustrate, it is known to produce hardened nickel base alloys by the production of a dispersed gamma prime phase formed by precipitation in the solid phase from the alloy matrix. Such gamma prime hardened alloys however, are uniform in the molten state. That is, the hardening phase, specifically Ni.sub.3 Al or a variant thereof, dissolves either in the solid or the liquid gamma phase matrix of the nickel base alloy and thus the alloy is castable by normal methods. However, if the nickel base alloy is hardened by a material such as thorium oxide e.g., the alloy known as TD nickel, or is hardened by refractory oxides such as yttrium oxide or oxidic compounds thereof and perhaps also hardened by gamma prime phase for example, as in the alloy known as INCONEL alloy MA 6000, the oxidic hardening phase will not dissolve in molten alloy. If the alloy is molten to any significant extent at any stage in its manufacture, the oxidic phase will be separated by gravity effects and not be effective for the purpose of hardening the alloy.
Alloys such as MA 6000 and many variants and improvements thereof, are normally made by a method called mechanical alloying. In this process, powders of the alloying ingredients are subjected to vigorous mechanical working in the presence of the desired oxidic dispersant until a significant fraction of saturation hardness of the alloying ingredients is obtained as well as an intimate combination of oxidic and metallic ingredients. The powders produced by mechanical alloying are then treated using powder metallurgical techniques specifically adapted for mechanically alloyed products and at no time during the manufacture of a mechanically alloyed object or during the life time of that object is the material melted. As stated before, if significant molten phase is produced the oxidic dispersion imparting strength to the mechanically alloyed object will be destroyed.
Reference is made to the text "The Art and Science of Growing Crystals" J. J. Gilman, Editor, J. Wiley & Sons Inc., 1963 particularly to pages 453 and 454. In this text it is disclosed that a single crystal seed can be isolated in a metal bar by cutting and that this single crystal seed can be grown epitaxially by gradient annealing to provide a single crystal in the bar.